1. Technical Field
This invention relates to technology for assembling and operating machine tool spindles using rolling bearings (at least certain of which are preloaded), and, more particularly, to techniques for verifying the accuracy of pre-loading and/or other settings of the bearings.
2. Discussion of the Prior Art
Properly assembled machine tool spindles, such as for boring and milling, have a designed axial bearing stiffness, which, if adhered to, provides a useful life of at least about 5000 hours before maintenance steps are undertaken to readjust the spindle bearings. Unfortunately, common assembly problems do affect bearing preloads, such as: (i) improper fitting or misalignment of either the outer bearing race rings into the housing bore or the inner bearing race rings onto the spindle shaft, (ii) presence of foreign particles in or about the race rings, and (iii) operator skill in wrongly controlling compression to obtain the preload as a result of torquing a locknut. If such problems are not detected prior to putting the spindle into service, it will have a useful life shortened to 25 hours or less, and often irreparable damage is done requiring replacement of bearings or other components.
What is needed is a method that enables plant personnel to quickly test and determine bearing preload conditions and other bearing settings to see if they all fall within specific designed parameters, thus preventing premature spindle failure and enhancing spindle reliability.
The inventors are unaware of any known method to perform verification of proper bearing preloads prior to the spindle being used in service. Commercial manufacturers of spindles may only subject their spindles to a test using an axially applied hydraulic force that moves the spindle shaft to test whether the outer race rings have been properly locked up to the housing while the spindle housing is fixed or clamped. This test gives no information or indication as to the accuracy of preload of the spindle bearings.
Machine tool spindle assemblies have essentially two masses which are interconnected by a compressed spring in the form of rolling bearing sets. An outer mass is constituted essentially of the spindle housing along with any outer bearing races, while an inner mass is constituted of a solid spindle shaft with any secured inner bearing races. The invention has discovered a simple technique for quickly and reliably verifying bearing functionality and determining whether the bearing preloads are within a designed range. The technique relies upon supporting the outer mass on non-resonating flexible members, such as cloth straps or a soft foam; the outer mass is essentially environmentally isolated. A natural vibrating frequency of the inner mass is induced by sharply striking the end of the spindle shaft desirably in a centered axial direction. The tone or frequency of the axially oscillating inner mass moves in opposition to the suspended non-moving outer mass as modified by the degree of axial stiffness imparted through the preloaded bearings located between the masses. The induced frequency is immediately measured and used as a tool to verify the functionality of the spindle assembly.
The method steps of this invention, in one aspect, comprises: (a) assembling a machine tool spindle having an outer housing rotatably supporting an inner spindle shaft about a spindle axis by use of rolling bearing sets, at least one set being compressed along the spindle axis to promote a bearing preload force between the housing and shaft; (b) suspending the housing on a non-resonating flexible support with the spindle shaft essentially isolated from the housing with freedom to oscillate as modified by the preloaded rolling bearing set; (c) sharply striking the spindle shaft along the spindle axis to induce an oscillating frequency in the shaft that is affected by the degree of bearing preload of such at least one bearing set; and (d) measuring the induced frequency and comparing it to a known frequency analysis of such type of spindle assembly, the frequency analysis being correlated to varying bearing preloads so that the comparison will indicate spindle bearing functionality.